The present invention relates generally to network communications systems, and more particularly, to a system and method for providing clock synchronization of source and destination clocking functions, for ATM variable bit rate traffic operating under the AAL2 and AAL5 layered protocols.
The modern communications era has brought about a tremendous proliferation of wireline and wireless networks. Computer networks, television networks, and telephony networks in particular are experiencing an unprecedented technological expansion, fueled by consumer demand. The ever-increasing need for transportation, due in part to the expansion of the world-wide market and the popularity of suburbia, has led to an increased use of automobiles and airplanes for business and pleasure. The desire to maintain the ability to communicate, even while away from the home or office, has driven the wireless communication market to a large extent. One response to this demand was the mobile/wireless telephone network.
The demand by consumers all over the world for mobile communications is expanding at a rapid pace and will continue to do so for at least the next decade. Over 100 million people were using a mobile service by the end of 1995, and that number is expected to grow to 300 million by the year 2000. Several factors are contributing to the exciting growth in the telecommunications industry. For example, a combination of technology and competition bring more value to consumers. Phones are smaller, lighter, have a longer battery life, and are affordable now for the mass market. Operators are providing excellent voice quality, innovative services, and roaming across the country or world. Most important, mobility is becoming less expensive for people to use. Around the world, as well as in the United States, governments are licensing additional spectrum for new operators to compete with traditional cellular operators. Competition brings innovation, new services, and lower prices for consumers.
Cellular telephone communications systems allow users of cellular telephones to be connected to other cellular telephone users, as well as being connected to the conventional landline Public Switched Telephone Network (PSTN). Cellular telephones work by dividing geographical areas into xe2x80x9ccellsxe2x80x9d. Each cell includes a base station, which typically contains a transceiver, antenna, and dedicated lines to a Mobile Telephone Switching Office (MTSO) or Mobile Switching Center (MSC). Adjacent cells utilize different radio frequencies in order to prevent interference between the adjacent cells.
Many standards exist for public mobile systems. One such standard is the Global System for Mobile communications (GSM), which is accepted in Europe and several countries outside Europe as the international standard for modern public mobile systems. GSM is an open system that can be used for both voice and data, and uses digital transmission for voice and signaling. GSM systems also use time-division multiplexing (TDM) that can offer many channels on one frequency.
A variety of data transmission technologies can be used in connection with mobile communications. One transmission technology currently being explored in connection with the present invention is asynchronous transfer mode (ATM) technology as it is used in connection with voice and data transmission in mobile system architectures such as GSM. ATM is a cell-based switching and multiplexing technology designed to be a general-purpose, connection-oriented transfer mode for a wide range of services.
ATM is a transfer mode that uses information xe2x80x9ccellsxe2x80x9d as the means of transferring information. The cell has a fixed length of 53 octets, which includes a 5-octet header and a 48-octet payload. The header includes a Virtual Path Identifier (VPI) and Virtual Channel Identifier (VCI) which identify the cell destination. A transmission path contains one or more virtual paths (VP), while each virtual path may contain one or more virtual channels (VC). The VPI together with the VCI identify the virtual circuit by which ATM switches route the call from source to destination. Other fields in the cell header include the Generic Flow Control (GFC) which allows a multiplexer to control the rate of an ATM terminal, and the Payload Type (PT) field that indicates whether the cell contains user data, signaling data, or maintenance information. The Cell Loss Priority (CLP) bit in the cell header indicates the priority of the cell relative to other cells, and the Header Error Check (HEC) field detects and corrects errors in the header.
In a communication system such as a Base Station System (BSS), a need to interface dissimilar technologies may arise. For example, a cellular network incorporating ATM technology may at some point require interfacing to a public switched telephone network (PSTN) or mobile switching center (MSC) that implements pulse code modulation (PCM). PCM refers to the traditional digital modulation method that encodes a voice signal into an eight-bit word representing the amplitude of each pulse. More specifically, the speech signal is sampled at 8 khz, and each sample is encoded using an 8-bit code to provide 8,000 samplings at 8 bits per sample, resulting in a 64-Kbps bit stream. Because of the dissimilarity between ATM and PCM information transfer technologies, conversion and synchronization may be required.
For example, an ATM-based BSS is designed to work in a cellular environment having an arrangement of cells, each cell having a base transceiver station (BTS) (also referred to as a base station or BS). Cells when grouped together form a cluster, and the MSC is connected to all of the base stations in a cluster. Alternatively, a base station controller (BSC), which is a modular switching platform, can perform the connection and traffic concentration between a number of BTSs and the MSC. A BSC also controls the basic functionality of those BTSs connected to it. Each BTS manages radio traffic with multiple mobile stations (MS). In sum, the BSS is a system of BTSs and BSCs which is viewed by the MSC through a single interface.
In such an ATM-based BSS, a transcoder (TC) is an inter-working function (IWF) between the ATM and the PCM environments. An IWF allows interoperation between a native protocol and an ATM-based end device. The PCM environment can be a PSTN or a MSC depending on the phase of the ATM-based GSM network evolution. A TC is a network element which provides the conversion between GSM-coded voice signals and the standard PCM 64-Kbit/sec signal used in the MSC and PSTN.
However, the interface between an ATM-based BSS and the PCM-based MSC (or PSTN) can bring about some synchronization concerns. The principal concern can be described as follows. The transmitting clock of the BTS operates at the frequency fBTS, and the receiving clock of the TC operates at the frequency fTC. Because the transmission is asynchronous, the TC will not know the exact frequency of the BTS transmitter. If fBTS is slightly higher than fTC and no discontinuous transmission (DTX) is used in the ATM, the receiving buffers of the TC may eventually overflow. Audible xe2x80x9cclicksxe2x80x9d will occur on the telephone connected to the PCM when a TRAU frame is discarded because of the overflow in the buffer. This holds true for communications in the opposite direction as well.
In order to avoid this overflow condition, the PCM interface of the IWF should be synchronized with the PCM component of the BTS. In many cases, the BTS will have some PCM-oriented components in order to synchronize with mobile stations, as well as an ATM interface.
Synchronization has been accomplished in various manners. If the ATM switches can support propagating a physically transmitted clock from the PSTN to the BTS, such as by way of plesiochronous digital hierarchy (PDH) or synchronous digital hierarchy (SDH) frames, synchronization can be accomplished.
Where the ATM switches do not support such physical clock propagation, other methods must be used. For example, the clock frequency can be xe2x80x9cdeducedxe2x80x9d in the BTS in some fashion. Some methods of deducing the sender""s clock frequency include investigation of the mean arrival rate of incoming ATM cells, and the investigation of the level of the buffer for the incoming ATM cells.
Investigation of the mean arrival rate of incoming cells involves the source sending cells at time tA where the destination knows the cell should arrive at time tB. The time difference tBxe2x88x92A is then used to adjust the clock. In ATM networks, the cell delay variation (CDV) can be diminished with the use of the median of the arrival time of a predetermined number of cells; for example ten cells. This method may be applicable in ATM-based GSM systems where one-to-one mapping is used.
Another investigation technique used to deduce the clock frequency in the BTS is to measure the size of the buffer for incoming cells. The object of this method is to keep the size of the incoming cell buffer at some relatively constant level C. If the buffer begins to fill, the clock frequency is increased until the cell buffer migrates back towards the level C. Similarly, if the buffer begins to empty, the clock frequency is decreased until the cell buffer migrates up towards the level C.
In another method, the synchronous residual timestamp (SRTS) used in connection with AAL1 can be used to deduce the clock frequency in the BTS. Before discussing this method, an understanding of the ATM Adaptation layers (AAL) is required.
ATM networks require the use of communication protocols, as is true in any network. A protocol is a formal set of conventions governing the formatting and relative timing of message exchange between two communication systems. For one computer to communicate with another, each must be capable of understanding the other""s protocol. In the computer and communications arena, protocols are defined through protocol architectures, and are modeled in a layered fashion with lower layer protocols providing services to the next higher layer. The protocols between terminal and network are referred to as lower layer protocols, while network protocols are used for the creation of the connection from switch to switch within the network itself. Higher layer protocols are protocols between two terminals, and the information is passed transparently by the network from one terminal to the other. The reference model for Open Systems Interconnection (OSI) is a standard of the International Organization for Standardization (ISO), and is perhaps the most generic protocol architecture.
The OSI reference model consists of a seven-layer model. Described generally, the physical layer is the lowest layer and provides access to the transmission medium and includes rules for the transmission of bits between source and destination. The next layer, the data link layer, is concerned with the transmission of frames of data between devices, and provides error detection/correction, multiplexing, and flow control. The network layer protocols provide end-to-end addressing and flow control. This layer accepts messages from higher layers, separates them into packets, routes them to the destination through the physical and data link layers, and reassembles them in the same form in which the source delivered them. The higher layers include the transport, session, presentation, and application layers. The transport layer provides multiplexing onto the network layer while the session layer establishes a connection between end systems. The presentation layer manipulates data into different forms for the highest layer, the application layer.
A layered protocol architecture directed to ATM technology has evolved from the OSI reference model. The ATM model is commonly referred to as the B-ISDN/ATM (broadband integrated services digital network) protocol reference model. This model comprises a physical layer (PHY layer) that corresponds to the OSI reference model layer 1, and an ATM level and ATM Adaptation Layer (AAL). The ATM level in connection with part of the AAL corresponds to OSI reference model layer 2 (data link layer). The ATM layer defines virtual paths and virtual channels, performs multiplexing, switching, and control actions based upon information in the ATM cell header, and passes cells to, and accepts cells from, the ATM Adaptation Layer (AAL). The AAL passes Protocol Data units (PDUs) to, and accepts PDUs from, higher layers. PDUs may be of variable length, or may be of fixed length different from the ATM cell length.
The physical layer includes two sublayers, referred to as the physical medium (PM) sublayer and the transmission convergence (TC) sublayer. The PM sublayer provides for the actual clocking of bit transmission over the physical medium. The TC sublayer converts between the bit stream clocked to the physical medium and ATM cells. The TC maps transmitted cells into the Time Division Multiplexing (TDM) frame format, and delineates individual received cells. The TC also generates the HEC on transmit for use in correcting and detecting errors, and performs cell rate decoupling by sending idle cells when the ATM layer has not provided a cell.
The ATM layer constructs the ATM virtual paths (VPs) and Virtual Channels (VCs) according to the VPI and VCI respectively in the cell header. The ATM layer also performs a variety of other functions, including cell construction, header validation, cell multiplexing and demultiplexing, and generic flow control.
The AAL is divided into the Convergence Sublayer (CS) and the Segmentation And Reassembly (SAR) sublayer. The CS sublayer is further subdivided into Service Specific (SS) and Common Part (CP) components. While the SSCS sublayer is optionally implemented, the CPCS must always be implemented along with the SAR sublayer. Generally, AAL layers 1-4 are defined to map to four different service classes defining timing relations, constant versus variable bit rates, and whether the connection mode is connection-oriented or connectionless.
As previously described, a synchronous residual timestamp (SRTS) has been in connection with AAL1 for synchronization purposes. AAL1 uses one octet of every cell payload to support unstructured circuit transport. A timing recovery functionality within AAL1 is necessary to maintain the bit timing across the ATM network and to avoid buffer overflow/underflow at the receiver. The SRTS concept is based on the source and destination having a very accurate frequency clock of frequency fn that represents the network clock frequency. The SRTS is a four-bit field in the AAL1-PDU (Protocol Data Unit) that establishes the residual portion between the source and network clocks. The signal has a service clock frequency fx with the objective being to pass sufficient information via the AAL1 so that the destination can reproduce this clock frequency with high accuracy. The network reference clock fn is divided by x such that 1xe2x89xa6fnx/fsxe2x89xa62. The source clock is divided by N to sample the 4-bit counter Ct driven by the network clock fnx once every predetermined number of bits generated by the source (e.g., every 8th ATM cell, or every 47*8*8=3008 bits). This sampled counter output is transmitted as the SRTS in the SAR PDU.
However, while there is a network clock available in the ATM network, the SRTS method described above does not account for the fact that the clock of the voice circuits of the BSS is not connected to the clock of the ATM network. AAL1 is not suitable for transmitting packet-oriented compressed voice data, as it is used exclusively for constant bit rate (CBR) traffic. The SRTS in AAL1 can not exploit the benefits of the Voice Activity Detection (VAD) function, i.e., variable bit rate (VBR) traffic. Therefore, it would be desirable to provide for the transmission of packet-oriented compressed voice data, such as ATM voice data, in a variable bit rate fashion while providing for synchronization between the source and destination. The present invention provides a solution to this and other shortcomings of the prior art, and offers other advantages over the prior art.
The present invention is directed to a system and method for transmitting, via ATM technology, synchronous timestamp information using ATM adaptation layers 2 (AAL2) and 5 (AAL5) between at least two ATM interfaces.
In accordance with one embodiment of the invention, a method is provided for synchronizing variable bit rate traffic between a source and destination in an Asynchronous Transfer Mode (ATM) network. The method includes designating a variable bit rate ATM Adaptation Layer (AAL) protocol to define ATM cell traffic flow from the source to the destination. A synchronous residual timestamp (SRTS) value indicative of the timing of a source clocking function is encoded and transmitted with an ATM cell using a variable bit rate AAL, such as AAL2 or AAL5. The ATM cells and associated SRTS values are received and decoded at the destination ATM interface, and the destination clock is synchronized with the source clock by modifying the destination clock according to the SRTS values.
In accordance with a more specific embodiment of the invention, an AAL5 protocol is designated to define the ATM cell traffic flow from the source to the destination. Transmission of the SRTS values with the ATM cells includes creating an SRTS field in an AAL5 Common Part Convergence Sublayer Protocol Data Unit (CPCS-PDU) trailer to occupy at least a portion of the SRTS value. A sequence number (SN) field in the AAL5 CPCS-PDU trailer is created to occupy a sequence number corresponding to a sequence and completion status of segmented portions of the SRTS value. The CPCS-PDU, including the sequence number and the segmented portion of the SRTS value from the CPCS-PDU trailer, is transmitted using the AAL5 protocol.
In accordance with another specific embodiment of the invention, an AAL5 protocol is designated to define the ATM cell traffic flow from the source to the destination. Transmission of the SRTS values with the ATM cells includes creating an SRTS field in an ATM payload section of the AAL5 Common Part Convergence Sublayer Protocol Data Unit (CPCS-PDU) to occupy at least a portion of the SRTS value. A sequence number (SN) field in the ATM payload section of the AAL5 CPCS-PDU is created to occupy a sequence number corresponding to a sequence-and completion status of segmented portions of the SRTS value. The CPCS-PDU, including the sequence number and the segmented portion of the SRTS value from the ATM payload section, is transmitted using the AAL5 protocol.
In accordance with another specific embodiment of the invention, an AAL2 protocol is designated to define the ATM cell traffic flow from the source to the destination. Transmission of the SRTS values with the ATM cells includes creating an SRTS field in an AAL2 Common Part Sublayer (CPS) packet to occupy at least a portion of the SRTS value. A sequence number (SN) field in the AAL2 CPS packet is utilized to occupy a sequence number corresponding to a sequence and completion status of segmented portions of the SRTS value. The CPS packet, including the sequence number and the segmented portion of the SRTS value from the CPS packet, is transmitted using the AAL2 protocol.
In accordance with another aspect of the invention, a cellular communications system for communicating voice information over a core network using Asynchronous Transfer Mode (ATM) technology and variable bit rates (VBR) is provided. The system includes at least one a base transceiver station (BTS) to communicate with at least one mobile station (MS), the BTS having a first ATM interface coupled to the core network. An Inter Working Function (IWF) having a second ATM interface is coupled to the core network to communicate with the first ATM interface in the BTS via the core network. Processing means integral to the BTS and IWF are provided for synchronizing variable bit rate traffic between the first and second ATM interfaces, where the synchronization means includes means for designating a variable bit rate ATM Adaptation Layer (AAL) protocol to define ATM cell traffic flow between the first and second ATM interfaces. The synchronization also includes means for generating a synchronous residual timestamp (SRTS) value indicative of the timing of a source clocking function, and for transmitting the SRTS values with the ATM cells using the variable bit rate AAL. The ATM cells and corresponding SRTS values are received at a destination ATM interface, and the destination clocking function is synchronized to the source clocking function using the SRTS values.